Powder metallurgy



United States Patent O M 3,410,684 POWDER METALLURGY Leon J. Printz,Detroit, Mich., assignor to Chrysler Corporation, Highland Park, Mich.,a corporation of Delaware No Drawing. Filed June 7, 1967, Ser. No.644,088 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE A process forproducing sintered porous metal objects through the compacting of powdermetals at low pressure. A fatty acid and a wax are blended with thepowdered metal prior to compacting so as to form a green briquette ofsufficient firmness and cohesiveness to withstand normal handlingwithout fracture or crumpling. Prior to sintering, the green briquetteis placed under reduced pressure and heated to remove the fatty acid andwax components, thereby avoiding dissociation of these materials atsintering temperatures into substances which react with the powder metalto adversely affect the chemical and physical properties of the desiredproduct. The green briquette is then sintered after removal of the fattyacid and wax.

BACKGROUND OF THE INVENTION The present invention relates to porousmetal objects and to a process for their manufacture involving thecompacting 'and sintering of powder metal. Porous sintered metalarticles such as bearings have long been known and have found wideacceptance due to their lubricating effectiveness when impregnated witha lubricant such as oil. Moreover, porous aluminum bearings are findingincreasing use due to their excellent heat conductivity, low weight andstrength. Accordingly, the balance of the discussion of this inventionwill be with respect to the manufacture of sintered aluminum andaluminum alloy metal, although it will be appreciated that the inventionis applicable to other metal systems such as bronze and copper.

While it is recognized that porous sintered aluminum is an excellentmaterial from which to fabricate bearings, it is also known, that thebearing performance is largely dependent upon the quantity of oil orother lubricant which can be incorporated into the bearing and the easewith which the lubricant can move to the load bearing surfaces.Understandably, the oil absorption and flow is primarily a function ofthe density of the metal bearing, and it is known that high densitybearings, that is, those formed at higher compacting pressures,generally are shorter lived than those formed at lower pressure sincethey are less porous, contain less oil and offer fewer oil flow paths.Accordingly, certain manufacturers engaged in the powder metal bearingart attempt to provide a highly porous sintered metal bearing by using avery low compacting pressure to briquette the powder metal. Unfortunatelthis approach has many problems stemming from the fact that metals, andaluminum in particular, are coated with a protective oxide film whichinhibits formation of satisfactory metal to metal bonds. Accordingly,when compacted under low pressure, the resulting green briquette doesnot have sufficient strength and cohesiveness to permit normal handlingduring the subsequent sintering and sizing manufacturing operations.Therefore, a high amount of scrappage is encountered unless extremelydelicate handling measures are used. Obviously, neither situatio'n'iscompatible with high volume, low cost production.

3,410,684 Patented Nov. 12, 1968 To avoid the above difficultiesassociated with the use of low briquetting or compacting pressures, manyin the art have turned to the use of high compacting pressures, i.e., 30to tons per square inch. The use of such high pressures, however, causesmany other problems which largely offset the advantages secured by theincreased cohesive strength of the green briquette produced thereby. Forexample, the use of high pressure has been found to cause the metal tostick to the die wall thereby requiring the need of costly equipment toeject the briquette from the die. Moreover, the high forces generatedduring ejection have been known to shatter or break away portions of thebriquette thus destroying the accuracy of the configuration.Accordingly, the art is repleat with various methods for lubricating thewalls of the die to prevent such sticking. Such methods are generallycostly and necessitate frequent die cleaning. A more serious problemlies in the fact that high briquetting pressures generally result inhigh density bearings with low oil absorption capacity. In order toprovide for increased oil absorption, a number of methods have beendeveloped for increasing porosity through reduction of the metal oxidecomponent of the bearing. For example, US. 2,133,761 teaches to addcopper oxide to the green briquette and then reduce the oxide duringsintering so as to provide voids in the bearing. However, such oxidereduction techniques are costly and dangerous since they generallyrequired using hydrogen at high temperatures which is an extremelyhazardous procedure. Accordingly, it is seen that the use of highbriquetting pressures does not provide a commercially attractive processdue to the cost of large die and die ejecting equipment and the need forhazardous reduction procedures to create the necessary bearing porosity.

SUMMARY An object of the present invention, therefore, is to provide animproved porous metal object by the metallurgical techniques involvingcompacting and sintering of powder metal.

A further object is to provide an improved method of making low densitysintered metal objects which overcorres prior difficulties and which issimple and effective.

A particular object of this invention is to provide a method ofpreliminary compacting, without the use of high pressures, aluminummetal powder briquettes which are adapted to being worked byconventional processes.

Other objects and advantages of the pre ent invention will becomeapparent from a further reading of the description and the appendedclaims.

This inve tion is based on my discovery of certain agents which enablethe production of strong, cohesive green powder metal briquettes throughthe use of low compacting pressures and, if high compacting pressure isemployed, eliminate the need for metal oxide reduction to providesuitable porosity and lubricant absorption properties.

The foregoing results are achieved by first blending the powder metal,prior to compacting, with a fatty acid and a Wax so as to form a mixtureof powder metal, fatty acid and wax and then compacting the mixture intoa green briquette of the desired configuration. The green briquette isthen vacuum heated to remove the fatty acid and wax components prior tosintering of the briquette. It is essential that substantially all ofthe fatty acid and wax be removed so as to prevent their dissociationand reaction at sintering temperatures with the metal powder, since suchreaction has been found to seriously adversely effect the chemical andphysical properties of the end product. Finally, after such removal, thegreen briquette is sintered and treated to produce the desired result.

DESCRIPTION The aluminous metal powders which can be employed in thisinvention may be of the flake or atomized type and the selection of theform of powder and particle size is dependent upon the use andperformance requirements of the end product. While a wide variation inparticle size is permissible, the particles should not be larger thanwill pass through a 35 mesh screen (Tyler Sieve Series). In general, itis desirable to utilize powders of a fine mesh size as in the range of200, 325 mesh and mixtures of different sizes, as is well known in theart, are frequently advantageous in securing certain properties. Themetal powder can consist of low purity aluminum, for example 99 percent,up to the highest purity obtainable, or particles of aluminum basealloys such as are formed by dissolving the alloying metal in moltenaluminum, or a mixture of aluminum and the desired alloying elementssuch as zinc, copper, manganese, tin, lead and magnesium and siliconwhich are 'commonly employed in the aluminum-alloy art. Excellentaluminum based bearings have been produced in accordance with thisinvention from an elemental alloy mixture consisting of, based onweight, 25% copper, 15% tin, 4 lead, 01.5% magnesium and the remainderaluminum. In general, aluminum bearings should contain from about 75 to95 weight percent of aluminum, the remainder essentially being an alloymaterial. However, where extremely small particle sizes are employed,for example 400 mesh, the aluminum content of the bearing can be as lowas 50 weight percent.

Prior to preparing the initial green briquette, the metal powder isblended with a fatty acid and a wax. The fatty acid component shouldcontain at least about 12 carbon atoms and can be either saturated orunsaturated. Preferably, the fatty acid contains from about 12 to 22carbon atoms and mixtures of such acids can be used. Examples ofsuitable acids include, lauric acid, palmitic acid, margaric acid,tridecanoic, stearic acid, oleic acid, brassidic acid, arachidic acid,linoleic acid, behenic acid, erucic acid, linolenic acid, elaidic acid,eleostearic acid, lichemic acid, ricinoleic acid, palmitoleic acid, andpetroselenic acid. The commercially available flake or powdered form ofthese acids can be used in this invention. It has been found that thequantity of acid needed in the process of this invention is dictated bythe configuration and metal composition of the green briquette and thatthe suitable amount for any given application can be determined byroutine experimentation by one of skill in the art. In general, goodresults have been obtained when the mixture to be compacted containsfrom about 0.2 to 2 weight percent of the fatty acid.

The primary criteria for selection of a suitable wax for use in thisinvention are that it act as a good binder for the metal powder so as toprovide substantial cohesive strength in the green briquette and that itbe volatilizable at a temperature not exceeding about 345 C. Naturallyoccurring animal and vegetable waxes as well as synthetic waxes whichhave a melting point up to about 150 C. have been found to meet theserequirements. Representative of waxes which can be employed in thisinvention are the ester reaction products of high molecular weight fattyacids, such acids having from about 12 to 34 carbon atoms, withcompounds containing at least one active hydrogen atom. The term activehydrogen atom refers to hydrogen which, because of its position in themolecule, displays activity according to the Zerewitinoff test asdescribed by Kohler in I. Am. Chem. Soc. 49 3181 (1927). The activehydrogen atoms are generally attached to oxygen, nitrogen or sulfur suchas OH, SH, NH, NH CONH CONHR where R represents an organic radical, SOOH, SO NH or CSNH and may be part of aliphatic, aromatic, cycloaliphaticor mixed type compounds. Typical of many active hydrogen containingorganic compounds which are useful are alcohols such as cetyl alcohol,ceryl alcohol, n-octadecyl alcohol, montanyl alcohol and myricylalcohol, and polyhydric alcohols such as ethylene glycol, diethyleneglycol and polyethylene glycol. Other representative compounds includenonoethanolamine, sulfonilamide, propylenediamine and ethylenediamine.Good results are obtained when the wax is an ester reaction product of afatty acid having from about 12 to 34 carbon atoms and an alcohol havingone or two hydroxyl groups. Examples of such materials are: carnauba waxhaving a melting point of about 87 C. and which is a mixture of theesters of the normal alcohols and fat acids having even numbers ofcarbon atoms from 24 to 34; beeswax which has a melting point of 60-82C. and whose composition resembles a carnauba wax except it is mainlycomposed of 25 and 28 carbon atom acids and alcohols; and spermacetiwhich is mainly cetyl palmitate and which melts at 4247 C. Equally goodresults are also obtained from synthetic waxes such as the amides,nitriles and amines of the higher fatty acids, all of which are familiarmaterials to those in the art. The amount of wax which is required isvariable but it has been found that an amount in the range of about 0.1to 1 weight percent is generally satisfactory.

The method by which the powdered metal is mixed with the fatty acid andwax components is not material provided that a substantially uniformmixture is obtained. Thus any form of mixing can be employed, such ashand mixing or any of the mechanical methods for uniformly mixingpowdered materials. After blending, the mixture of powder metal, fattyacid and wax is compacted into a green briquette. Briquettes having ahigh degree of firmness and cohesiveness have been formed without theneed of external heating through the use of relatively low compactingpressures in the range 2 to 10 tons per square inch. Higher pressurescan be employed if desired. Since this invention permits the use ofrelatively low compacting pressures, such pressures can be exerted byconventional and rather simple means and thus obviate the need forspecial dies or presses and high pressure equipment. This represents aconsiderable economy in the production of powder compacts.

.Prior to heating the green briquette to sintering temperature, it isnecessary that substantially all of the fatty acid and wax components ofthe briquette be removed since it was found that these componentsdisassociated at temperatures above about 345 C. into constituents whichadversely affect the chemical and physical properties of the sinteredproduct. Accordingly, after briquetting, the green briquettes aresupported on trays and placed in a vacuum chamber which is thenevacuated, for example to a partial pressure of about 50 to 200 microns.After the chamber has been evacuated to the desired vacuum, or with thestart of the evacuation step, heat is then applied to the chamber toraise the temperature of the green briquettes to an elevated temperatureabove the melting point of the wax but not exceeding about 345 C. Duringthe heating, a gas, preferably an inert gas, is bled into the chamberand into direct contact with the briquettes so as to sweep the volatizedfatty acid and wax components from the chamber. Bleeding of the gas intothe chamber will cause the pressure in the chamber to rise, and it hasbeen found that to insure essentially complete removal of the fatty acidand wax from the briquettes, that is, at least about percent removal,the chamber should be kept at a partial pressure not exceeding about3500 microns. In the practice of this invention excellent results havebeen obtained when chamber pressure was in the range of about 200 to3500 microns and preferably from about 500 to 1500 microns.

The sweep gas should be bled into the chamber at a rate sufiicient toremove the volatized fatty acid and wax components from the vicinity ofthe briquettes and to prevent back diffusion of these components.Experiments as to the proper rate have shown that excellent results areobtained, for example, at a sweep gas flow rate in the range of about1000 to 1500 cubic centimeters (c.c.) per minute per briquette at achamber pressure of 1000 microns. Thus, for example, if the chamberoperating at a partial pressure of 1000 microns contained briquettes, aflow rate in the range of 10,000 to 15,000 cc. per minute would be used.In addition to removing the volatiles, the sweep gas provides thedesired results of a more uniform heating of the briquette and it hasbeen found that any non-oxidizing gas can be used. As mentioned earlier,it is not necessary in the practice of this invention to provide forreduction of a metal oxide as in many heretofore known processes and,hence, there is no need to use hydrogen or ammonia which is an extremelyhazardous procedure atelevated temperatures. The preferred sweep gasesfor use in this invention are the inert gases such as nitrogen, argon,helium and the like.

The rate at which the briquettes are initially heated to volatizc thefatty acid and wax is not critical and it will be understood that thebriquettes can be placed in a chamber which is cool and the chamber thenbrought up to elevated temperatures, or they can be placed directly intoa chamber which is already at an elevated temperature. The briquettesshould be heated for a suflicient time to'permit essentially completevolatization of the fatty acid and wax and this can be easily determinedby analysis of the sweep gas coming from the chamber. For example, inproducing aluminum bearings by this invention, it was found that thefatty acid and wax were completely volatized in the minute periodrequired to heat the briquettes to a temperature of approximately 320 C.

After the fatty acid and Wax components have been removed, thebriquettes are heated to a suitable sintering temperature, generallyabove 500 C., which is determined primarily by the composition of thegreen briquette. Preferably, the sintering is accomplished by merelyincreasing the temperature in the chamber while continuing to employ asweep gas and partial vacuum. However, the sintering operation does notrequire the use of a sweep gas and partial vacuum and these conditionsneed not be employed. Likewise, the green briquettes upon removal of thefatty acid and wax components can be transferred to another furnace forsintering. To complete the fabrication of bearings, the sintered metalarticles are impregnated with oil as by immersing them in oil in avacuum chamber. The chamber is then evacuated until air no longer flowsout of the bearings and the chamber is returned to atmospheric pressureso as to allow oil to fill the bearing. The bearing is then coined.

The following examples illustrate the process of this invention and theproduct produced thereby, the invention however not being limited to thespecific details thereof.

EXAMPLE I Screen Analysis (Tyler Sieve Series) +65 Mesh +200 Mesh +325Mesh 325 Mesh Percent:

Copper 0 9 27 64 'I" .........r 0 1 3 96 Lead 0 1 3 96 Magnesium 0 0 298 Aluminum. 7 31 17 The aluminum had an apparent density of 1.1-1.3gms./cc., a chemical purity of 99.5% minimum and average particlediameter of 22-26 microns; the magnesium had an apparent density of0.5-0.6 gms./ cc. and a chemical purity of 99.8%; the copper had anapparent density of 0.9-1.1 gms./cc. and a chemical purity of 99.4%minimum; the tin had an apparent density of 2.50-3.60 gms./cc. and achemical purity of 99.5% minimum; and the lead had an apparent densityof 4.75-5.75 gms./cc. and a chemical purity of 99.5% minimum.

EXAMPLE II This example illustrates the preparation, using the materialsdefined in Example I, of porous aluminum bearings of the followingcomposition:

Material: Parts by Weight Aluminum 90.58 Magnesium 0.75 Copper 4.00 Tin2.67 Lead 2.00 Ethylene distearamide 0.25 Stearic acid 0.75

A 200 pound mixture of the above composition was prepared by adding 1.5pounds of stearic acid powder to 7.5 pounds of aluminum powder to form afirst mixture. This mixture was then stirred to form a uniform blend andpassed through a 40 mesh screen into a storage container. The stearicacid which was used had a sieve analysis of 99.5% through 30 mesh,through mesh; a titer of 147-148 R; an acid value of 198-203; an iodinevalue of 64-65; and was a stearic acid palmitic acid mixture containingabout 83% stearic acid.

Using the same procedure, a second mixture was pre pared by adding 0.5pound of ethylene distearamide to 5 pounds of aluminum powder. Thesefirst and second mixtures were then added to 181.15 pounds of aluminumpowder along with 8.0 pounds of copper, 4.0 pounds of lead, 5.35 poundsof tin and 1.5 pounds of magnesium. This mixture was then blended for 30minutes so as to provide for uniform distribution and had a density ofapproximately 1.3 gm./ cc. After blending, the mixture was placed into adie and compacted at a pressure of approximately 4 tons per square inchinto green briquettes. No external heating was used during compactionand the briquettes had a density of about 2.25 gm./cc.

The briquettes were then placed loosely into a basket which was thenplaced on removal shelves of a vacuum retort. Each basket containedapproximately 40 briquettes and 14 baskets were arranged in the retort.Vacuum pumps were then started to produce a partial vacuum or pressurein the retort of about 85 microns. Nitrogen was then bled into thechamber and the pressure in the retort rose to about 600 microns.Simultaneously with the start of the nitrogen flow a furnace was loweredabout the retort and the briquettes are brought to a temperature ofabout 315 C. in about 20 minutes. At the end of this period, analysis ofthe nitrogen sweep gas being exhausted from the retort showed that thestearic acid and ethylene distearamide had been removed from thebriquettes and the temperature of the briquettes was then raised to 545C. and held for approximately 30 minutes while continuing the flow ofnitrogen and maintaining the vacuum. At the end of this sinteringperiod, the furnace was raised and the retort was opened after reachinga temperature of about 70 C.

The thus produced sintered bearings, having a length of 0.75 inch, anoutside diameter of 1.0 inch and an inside diameter of 0.75 inch, werethen impregnated with a Terrestic grade oil, which is a highly refinedturbine oil having a high viscosity index and oxidation stability, undera vacuum of 26 inches of mercury. The bearings were then tested and thefollowing data obtained:

Density gm./cc 2.20/2.35 Oil content percent minimum 15 Compressivestrength lbs./ sq. in 12,000 Apparent hardness RH 40/50 PV performance"55,000

*P equals bearing load (pounds per square inch) and V equals shaftvelocity (feet per minute).

From the foregoing, it will be appreciated that the objects of thisinvention have been obtained. A low density bearing of excellentproperties can now be fabricated by low pressure compaction and withoutthe need for hazardous metal oxide reduction techniques.

I claim:

1. A method of producing porous metal objects by the sintering ofpowdered metal which comprises blending a powdered metal selected fromthe group consisting of aluminum, aluminum and metal .alloying mixtureand aluminum base alloys with a fatty acid having at least about 12carbon atoms and a wax having a melting point not exceeding about 150 C.so as to form a mixture of metal powder, fatty acid, and wax, compactingthe mixture into a green compact having a desired shape, substantiallyremoving all the fatty acid and wax from the green compact prior tosintering of the compact, and after such removal heating said compact tosintering temperature, said removal being effected by placing the greencompact in a chamber and evacuating the chamber to a pressure less thanatmospheric, heating the green compact to a temperature not exceedingabout 345 C. to volatize substantially all the fatty acid and waxcomponents of the compact, and bleeding a gas into the chamber in directcontact with the compact so as to sweep the volatilized fatty acid andwax from the chamber prior to heating said compact to sinteringtemperature.

2. The method of claim 1 wherein the fatty acid has from about 12 to 22carbon atoms and wherein the wax comprises an ester produced by theinterreaction of a fatty acid and a compound containing at least oneactive hydrogen atom.

3. The method of claim 1 wherein the mixture of powdered metal, fattyacid and wax contains from about 0.1 to 1 weight percent of wax and fromabout 0.2 to 2 weight percent of fatty acid.

4. The method of claim 1 wherein the chamber is at a partial pressure offrom about 200 to 3500 microns during heating of the green compact toeffect removal of said fatty acid and wax.

5. The method of claim 1 wherein the fatty acid is selected from thegroup consisting of lauric acid, palmitic acid, stearic acid, oleic acidand mixtures of the foregoing.

6. The method of claim 1 for producing a bearing which comprises forminga mixture comprising a powdered metal selected from the group consistingof aluminum, aluminum and metal alloying mixture, and aluminum basealloys, a fatty acid having from about 12 to 22 carbon atoms and a waxhaving a melting point not exceeding about 150 'C., said mixturecontaining about 75 to 95 weight percent of aluminum metal, about 0.1 to1 weight percent of wax and from about 0.2 to 2 weight percent of fattyacid, compacting said mixture and placing the green compact in achamber, evacuating said chamber to a partial pressure of from about 200to 3500 microns and heating the green compact to a maximum temperatureof about 345 C., and simultaneous with said heating bleeding an inertgas into the chamber in direct contact with the compact.

7. The method of claim 6 wherein the fatty acid is selected from thegroup consisting of lauric acid, palmitic acid, stearic acid, oleic acidand mixtures of the foregoing, and wherein the wax is selected from thegroup consisting of an ester produced by the interreaction of a fattyacid having from 12 to 34 carbon atoms with an alcohol having a maximumof two hydroxyl groups, amides of fatty acids having from 12 to 34carbon atoms, and mixtures of the foregoing.

8. The method of claim 6 wherein the fatty acid is stearic acid.

9. The method of claim 6 wherein the wax is ethylene distearamide.

10. An aluminum bearing produced in accordance with claim 1 wherein themixture comprises a minimum of about weight percent of aluminum, fromabout 0.1 to 1 weight percent of wax and from about 0.2 to 2 weightpercent of a fatty acid having from about 12 to 22 carbon atoms, whereinsaid mixture is compacted in a briquette die under a pressure of about 2to 10 tons per square inch to form a green briquette, wherein the fattyacid and wax components of the green briquette are removed prior tosintering of the green briquette by placing the briquette into a chamberand evacuating the chamber to a partial pressure in the range of about200 to 3500 microns while simultaneously heating the briquette to atemperature in the range of about 200 to 345 C. and bleeding an inertgas into the chamber in direct contact with the briquette so as to sweepthe volatilized fatty acid and wax from the chamber, and finallysintering the briquette by heating it to a temperature of at least about500 C.

References Cited UNITED STATES PATENTS 1,873,223 8/1932 Sherwood 752122,122,053 6/ 1938 Burkhardt 75224 X 2,276,453 3/1942 Bandur 75214 X2,386,544 10/1945 Crowley 75214 X 2,792,302 5/1957 Mott 75222 X2,928,733 3/1960 Wagner 75224 X 3,001,871 9/1961 Thien-Chi 75214 X3,185,566 5/1965 Galmiche 75212 3,266,893 8/1966 Duddy 75222 3,313,6224/1967 Potet 75214 X FOREIGN PATENTS 468,518 7/1937 Great Britain.855,203 11/1960 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

A. J. STEINER, Assistant Examiner

